Documentation hamburger button lacks visible focus indication

The hamburger menu button in v4 and upcoming v5 documentation on small screen lacks a visible focus indication (e.g. when navigating by keyboard)

incidentally, this would be the sort of thing that would ideally, in future, be tackled using :focus-visible (as you'd want some sort of focus outline, but ideally only when navigating with keyboard and not after clicking with mouse/tapping on touchscreen)

xref https://github.com/twbs/bootstrap/pull/30073#issuecomment-601054410